Torque sensors are well known in the art. Such sensors typically produce an electrical signal having an electrical characteristic that varies as a function of torque applied to a member being measured. The output signal from the torque sensor is typically used to control another device. Proper control of the device requires that the electrical characteristic of the torque sensor output signal be at a known quantity when no torque is applied to the member so that applied torque can be accurately measured.
One problem that exists in torque sensors is that its output signal may include an undesired electrical offset. The existence of the electrical offset could result in inefficient or inexact operation of a device which is being controlled in response to the output signal from the torque sensor. The causes of electrical offsets in torque sensors are numerous and diverse. Such electrical offsets could result from misalignment of the elements that make up the torque sensor, temperature effects on the elements, etc. The problem with electrical offsets in torque sensors is of particular concern in the art of vehicle power assist steering systems.
In a vehicle power assist steering system, a torque sensor is used to generate an electrical signal having an electrical characteristic that varies as a function of applied steering torque. A torque sensor for a vehicle power assist system typically includes a position sensor in combination with a torsion bar. The torsion bar resiliently connects two coaxial shaft sections, one shaft section connected to the vehicle steering wheel and the other shaft section connected to a steering member. The position sensor outputs an electrical signal having an amplitude which varies as a function of the relative rotational position between the two shaft sections. Thus, the output signal of the position sensor is indicative of applied steering torque.
The output signal from the position sensor is connected to an electronic control unit ("ECU"). The ECU controls a device, such as an electric assist motor, that provides power assist in response to the output signal from the position sensor. The amount of power assist is, therefore, controlled in response to the sensed steering torque. The proper control of the electric assist motor is dependent upon the accuracy of the output signal from the torque sensor. Due to manufacturing tolerances, misalignment of torque sensor elements can result in the electrical output signal from the torque sensor being offset and therefore, not indicative of the true applied steering torque. Also, temperature variations may effect changes in the output signal of the torque sensor.
U.S. Pat. No. 4,509,611 to Kade et al. uses a torsion sensing device which outputs an electrical signal indicative of the applied steering torque. Ideally, the output signal from the torque sensor is at a mean value when no steering torque is being applied. The torque signal increases or decreases from this mean value depending upon the direction of the applied steering torque. This '611 patent recognizes that a misalignment of the torque sensor elements results in a steady-state error in the signal being outputted by the torque sensor. The steady-state error is determined using a time averaging method which, in one embodiment, takes approximately seven minutes to complete. Once determined, the steady-state error is added to or subtracted from the actual measured torque sensor signal to establish an artificial zero torque reference (ZTR) signal. The long period of time needed to zero the torque sensor of Kade et al '611 is undesirable and presents an obstacle to practical use of the arrangement in a power assist steering system.